One existing method for assessing a degree of arteriosclerosis is to measure a pulse wave by compressing a measurement subject's site from outside and assess a degree of arteriosclerosis based on the measured pulse wave. For example, JP 2004-113593A (hereinafter Patent Literature 1) discloses a device including a cuff for measuring a pulse wave and a compression cuff for compressing a portion on the distal side. The device disclosed in Patent Literature 1 measures a pulse wave from the heart while compressing the portion on the distal side so as to separate an ejected wave ejected from the heart from a reflected wave from the bifurcation of the iliac artery and various portions of the artery. The device disclosed in Patent Literature 1 then determines a degree of arteriosclerosis based on indexes such as the difference and the ratio between amplitudes of the ejected wave and the reflected wave, and the difference between times of appearances of the ejected wave and the reflected wave.
In order to detect a degree of arteriosclerosis with high accuracy using the above technique, it is necessary to correctly detect the start point of a reflected wave that appears in a pulse wave. In view of this, for example, JP 2009-517140A (hereinafter Patent Literature 2) discloses a method for separating an ejected wave from a reflected wave using a blood pressure waveform of the aorta and estimated values of a blood flow waveform. FIG. 17A shows one example of a blood pressure waveform. It is considered that the blood pressure waveform of FIG. 17A is obtained by compositing the aforementioned ejected wave and reflected wave as shown in FIG. 17B. According to this method, either a pressure waveform estimated using a transfer function method from a pressure waveform measured in a peripheral artery in an upper body (e.g. a radial artery and a brachial artery), or a pressure waveform measured in a carotid artery, is used as an approximation of the blood pressure waveform of the aorta.
Note that the transfer function method is described in, for example, U.S. Pat. No. 5,265,011 (hereinafter Patent Literature 3). Furthermore, a triangle waveform is used as the blood flow waveform. The triangle waveform is formed by a base, from the start point of a rising edge to the end point of a falling edge of the blood pressure waveform, and a vertex represented by the peak of contraction of the heart, as described in Westerhof, B. E. et al., (Aug. 28, 2006), Quantification of wave reflection in the human aorta from pressure alone: a proof of principle, [online], retrieved May 19, 2010 from http://hyper.ahajournals.org/cgi/reprint/48/4/595 (hereinafter Non-Patent Literature 1).    Patent Literature 1: JP 2004-113593A    Patent Literature 2: JP 2009-517140A    Patent Literature 3: U.S. Pat. No. 5,265,011    Non-Patent Literature 1: Westerhof, B. E. et al. (Aug. 28, 2006). Quantification of wave reflection in the human aorta from pressure alone: a proof of principle, [Online], retrieved May 19, 2010 from http://hyper.ahajournals.org/cgi/reprint/48/4/595.